1. Field of the Invention
The present invention is directed to an implantable medical device and, in particular, to an implantable medical device that automatically triggers a safety mode when exposure to a static magnetic field and/or Radio Frequency (RF) field, e.g., produced by a Magnetic Resonance Imaging unit, exceeds predetermined threshold levels followed by a self testing mode once the magnetic field and/or RF field falls below the associated predetermined threshold level.
2. Description of Related Art
Magnetic Resonance Imaging (MRI) devices are widely used as a medical diagnostic tool primarily due to its non-invasive properties. MRI is based on the intrinsic property of atomic nuclei known as spin. Specifically, atomic particles in the nuclei spin about their respective axes thereby producing angular momentum. When an atom has an odd number of both protons and neutrons then an angular momentum is produced; otherwise, the angular momentum is zero when an even number of both protons and neutrons exist in the atom.
In addition, certain nuclei exhibit magnetic properties. Specifically, protons as a result of their mass, positive charge and spin produce a relatively small magnetic field (a magnetic dipole moment). When no magnetic field is present the direction of magnetic dipole moments of population of nuclei are randomly oriented. However, when subject to a relatively large external static magnetic field (e.g., along the z-axis), the proton will assume one of two possible positions, i.e., aligned substantially parallel or anti-parallel with the direction of the external magnetic field. Exposure of the proton to the external static magnetic field also causes the proton to precess (spin like a top) about their magnetic dipole moments at some frequency expressed by the following Larmor Frequency Equation:ω0=γ B0 
where,                ω0 represents the Larmor frequency;        γ is the gyromagnetic ratio representing the ratio between the angular momentum and the magnetic moment, wherein the gyromagnetic ratio is specific to each magnetically active nuclei; and        B0 is the strength of the external static magnetic field. Typically, MRI machines currently apply a static magnetic field in the range between approximately 0.7 T and 1.5 T, or even upwards of 3.0 Tesla. Even higher static magnetic fields are contemplated since the higher the field the better the resolution and definition of the image.        
Generally, hydrogen nuclei spin is used in MRI procedures due to its abundant quantities in the human body. The gyromagnetic ratio for hydrogen is 42.6 MHz/T. Accordingly, the Larmor frequency for hydrogen is approximately 30 MHz for 0.7 T and approximately 64 MHz for 1.5 T.
A proton with a magnetic moment can have one of two possible energy states I=±½. When subject to an external static magnetic field, the protons align themselves substantially parallel or non-parallel (anti-parallel) to the direction of the external static magnetic field. Those protons that are non-parallel (anti-parallel) have a slightly higher energy state than those that are parallel. While in the presence of the external static magnetic field, if the protons are irradiated with a Radio Frequency (RF) signal in the x-y plane at the Larmor frequency some of the lower energy parallel protons will absorb energy from the RF wave and rotate the magnetic dipole moment to a non-parallel (anti-parallel) orientation, referred to as magnetic resonance. Those protons that have rotated to the non-parallel (anti-parallel) orientation are now in a higher energy state. After the radiation is removed, some of the non-parallel (anti-parallel) protons will rotate back to its lower energy parallel orientation (equilibrium state) and release or emit a relatively small amount of energy at the Larmor frequency to the environment as an RF wave. Detection of this relatively low level energy emission is detected by sensors and constitute the signal of interest that is used as the basis for the image.
Patient's having implanted medical devices that undergo an MRI procedure may be subject to two different types of potentially deleterious effects. This is especially noteworthy considering the fact that MRI procedures typically range in time from approximately 20 minutes to 90 minutes, or even longer. Therefore, patient's will be subject to these effects over an extended period of time.
The first effect is that resulting from the required static magnetic field at relatively high levels. While undergoing an MRI procedure, exposure to such a relatively high magnetic field may potentially cause malfunctioning or disruption in an implanted medical device of a mechanical or electromechanical component thereof that is sensitive to such relatively high static magnetic fields. For example, exposure to the magnetic field may undesirably cease or alter operation of a motor comprising magnets and coils, an electro-magnetically actuated part (e.g., a valve of a drug delivery device), an actuator, a sensor or any other component of an implantable medical device that may be sensitive to such relatively high static magnetic fields as required during MRI procedures.
Another safety precaution is the effect due to the RF field. For instance, an RF field will generate a relatively high voltage if coupled to a conductive loop. Accordingly, the RF field may cause dysfunctions at the level of electronic components (e.g., RAM, EEPROM, Flash memories, CPU function, sensors, actuators) or at the level of the system (e.g., circuit, leads).
These deleterious effects are of particular concern to those patient's having implantable medical devices (e.g., drug delivery devices, pacemakers, defibrillators or stimulators) employing components (e.g., motors, valves, actuators, sensors, integrated circuits, memory chips or solid state sensors) that may be sensitive to the relatively high magnetic fields and/or RF fields produced during the MRI procedure. These adverse effects on implantable medical devices produced during MRI procedures has led the medical industry to recommend that patients having such devices forego MRI as a diagnostic imaging tool. In the event that a patient still chooses to undergo an MRI then the manufacturers of the electronically operated implantable medical devices typically advise the patient to have the implant parameters verified and reset after the procedure generally using an external reading unit that communicates by telemetry with the implant device. This verification or checking of the operation of the implantable medical device is impractical in that it does not alter or correct the potentially inaccurate operation of the device during the procedure and requires external reading instrumentation particular to the implant device by specially trained medical personnel to be present immediately following the medical procedure to verify the accuracy of its operation.
Recently research and development has focused on finding a solution for detecting when electronic implantable devices are subject to adverse interference from electromagnetic radiation (Radio Frequency radiation). U.S. Pat. No. 6,198,972 discloses an apparatus for limiting unwanted current induced by a significant level of an external signal such as a time-alternating electromagnetic field in a conductive loop used to deliver electrical stimulation to electrically excitable tissue. Such electrical stimulation for instance may be applied to brain tissue to reduce or suppress tremors, peripheral nerve tissue to promote blood circulation and for pain management. The electrodes are electrically connected by a conductive lead wire to an implantable pulse generator. A switch is operatively connected within the conductive loop between the two devices and is controlled by control circuits. The switch is turned on when the level of detected electromagnetic radiation is less than or equal to a predetermined threshold thereby closing the conductive loop and allowing the electrical stimulation to excite the tissue, whereas the switch is turned off and the conductive loop is opened when the detected electromagnetic field exceeds the specified threshold. Since the switch is turned off and the conductive loop is opened when the external electromagnetic field exceeds the specified threshold all operations or actions by the medical device cease during this period of adverse electromagnetic exposure until the switch is once again turned on and the conductive loop is closed when exposure ceases. For instance, no electrical stimulus can be applied during this time as a result of the cutoff of connectivity to the pulse generator. This patented device would therefore not be suitable for a medical device that must continue some type of operation or functionality during this interval of exposure. Furthermore, no testing is disclosed to verify that the stimulating electrodes are working properly once exposure has ceased and the switch changes to a closed state.
Another device is described in U.S. Pat. No. 5,877,630 in which the field strength and amplitude modulation of the electromagnetic radiation is detected. So long as the combined field strength and amplitude modulation of the detected electromagnetic radiation (Radio Frequency radiation) exceeds a predetermined threshold level, the operation of the electronic device is continuously tested. Specifically, a series of testing pulses are continuously generated and sent to the CPU of the electronic device so long as the combined signal of field strength and amplitude modulation of the detected electromagnetic radiation exceeds the predetermined threshold. A response from the CPU to each pulse is expected within a predetermined time interval as confirmation that the medical device is operating properly. Each time the CPU responds to a testing pulse, a countdown timer is reset. The timer counts down with each pulse signal that is not responded to by the CPU until the timer reaches zero whereupon an alarm may be sounded. Therefore, this device does not assume that the medical device will not operate properly simply because the device is subject to electromagnetic radiation that exceeds a predetermined threshold level. Instead, a second level of criteria is applied when the threshold level is exceeded, that is, testing the operation of the CPU in responding back to continuously generated test pulses. Only if both criteria are met, i.e., the radiation exceeds a threshold level and no response is received to the generated test pulse within a predetermined time period, is the medical device classified as working improperly. This patented device is inefficient in its expenditure of energy to generate the testing pulses during the entire time interval that the medical device is subject to electromagnetic radiation exceeding the predetermined threshold. In addition, the device fails to take into consideration the situation in which the CPU responds to the test pulse but nevertheless operation of the medical device has been altered or disrupted in other ways (e.g., timing frequency has been altered or data stored in memory has been corrupted, altered or erased) due to interference from exposure to the electromagnetic radiation. Yet another disadvantage is that no verification of operation of the medical device is conducted after its exposure to the electromagnetic radiation ceases, testing occurs only while the device is subject to the electromagnetic radiation exceeding the predetermined threshold level.
It is therefore desirable to develop an implantable medical device that automatically activates at least one of, preferably both, a safety mode triggering some modified or altered operation of some part of the implantable medical device when exposed to a magnetic field and/or RF field that exceeds acceptable predetermined thresholds. When exposure to the static magnetic field and/or RF field falls below the predetermined threshold, thereafter automatically switching from a safety mode to a self testing mode so as to verify that the medical device is operating properly.